GASTROPODS. 



393 



upon all sorts of pelagic life which surrounds them. They have no jaws, but are 

 furnished with a radula with seven rows of teeth like the tsenioglossate gastropods. 

 Some have shells capable of containing the retracted animal ; in others, the shell 





m 



A PELAGIC HETEUOPOD, Ptcrotrachca (iini. size). 



